This invention relates to compensating crystal oscillators for changes which occur to the crystal oscillators due to acceleration.
Crystal oscillators are widely employed in a variety of vehicles which are subject to extreme g-conditions. Due to the importance of the accuracy of the oscillator output, compensation schemes have been proposed which compensate for changes which occur to the oscillator under acceleration, both static (such as constant gravity) and dynamic (vibration).
U.S. Pat. No. 4,100,512 entitled "Crystal Oscilator Compensated Against Frequency Shift Due to Acceleration", issued on July 11, 1978 having as inventors Michel Valdois and Armand B. Dupuy discloses a crystal oscillator which is compensated for by utilizing an adjusting capacitance having at least one variable capacitor, such that the frequency shift in the resonator caused by g-effects is compensated by the frequency shift of the variable capacitor. The Valdois patent provides a complete description of the effects of acceleration upon crystal oscillator circuits, and the presentation in that patent is referenced as background material for the present invention.
The crystal oscillator described in the Valdois patent does not permit independent adjustment of the oscillator frequency without at the same time, unavoidably changing the degree of compensation of the complete network circuits. The oscillator may be provided with exact g-force compensation at its set frequency, but there is no ability to adjust it to a user's specific frequency requirement, without having an adverse effect on the desired acceleration immunity.
The applicant in the present invention authored an article entitled "Acceleration Sensitivity Compensation in High Performance Crystal Oscillators" which was presented at the Tenth Annual Precise Time and Time Interval (PTTI) Applications and Planning Meeting, November, 1978. This paper also presents excellent background material relating to the variations encountered in high performance crystal oscillators under acceleration conditions.
In this paper, several compensation schemes are identified in the first column of page 3 as well as on page 4. Although this article suggests that the compensator coefficient depends upon oscillator tuning, there is no presentation of an effective adjustable oscillator having complete acceleration compensation for each and every setting of the frequency tuning.
Sometimes it is desired that the user adjust the basic oscillator frequency as determined by the crystal resonator in order to adapt the oscillator to specific user needs. This might be encountered where the oscillator is used in a phaselock loop circuit or in other circuits in which tunable frequencies are required. One way of adjusting the crystal oscillator is to employ a variable reactance network in the resonator circuit. The variable reactance network may be adjusted to alter the basic frequency of the oscillator. An adjustment of a variable reactance network in the crystal resonator circuit also causes a shift in the static reactive value, so that conditions of network reactance which previously afforded the correct sensitivity for the acceleration-compensation element, now no longer are correct for exact compensation.
An object of this invention is to provide an adjustable reactance network which can exactly compensate for acceleration variations of a crystal resonator for all conditions of static reactance load which result from the necessity to adjust the frequency of the oscillator.
Another object of this invention is to provide such a compensating network which also minimizes centripetal rotational effects by packaging the elements of the apparatus accordingly.
Yet another object of this invention is to provide reactance compensation for second order and stray capacitance effects.
Another object of this invention is to provide such a compensating network employing a variable or selectable reactance which may be easily adjusted to provide a reactance null at zero effective acceleration while compensating for the acceleration sensitivity of the crystal oscillator.
Other objects, advantages and features of this invention will become more apparent from the following description.